Construction Method for Under Ground Stations - LRMTS

Being a Civil Engineer, I thought to also include the construction type and the methods to be adopted while the execution of this huge project.

Diaphragm walls are envisaged for the underground stations construction. The technique involves excavating a narrow trench that is kept full of an engineered fluid or slurry. The slurry exerts hydraulic pressure against the trench walls and acts as shoring to prevent collapse. Slurry trench excavations can be constructed in all types of soil, even below the groundwater table. Cast in place diaphragm wall is usually excavated under bentonite slurry.

The construction sequence of a diaphragm wall is illustrated in Figure

Diaphragm wall construction begins with the trench being excavated in discontinuous sections or "panels". Stop end pipes are placed vertically at each end of the primary panel to form joints for adjacent secondary panels.

Once the excavation of a panel is complete, a steel reinforcement cage is placed in the centre of the panel. Concrete is poured in one continuous operation through one or more tremie pipes that extend to the bottom of the trench. The tremie pipes are extracted as the concrete rises; however, the discharge end of the tremie pipe always remains embedded in the fresh concrete.
The slurry that is displaced by the concrete is reused for subsequent panel excavations. When the concrete sets, the end pipes are withdrawn. Similarly, secondary panels are constructed between the primary panels to create a continuous wall. The finished wall may be cantilever or require anchors/props for lateral support.

The following summarise the diaphragm wall advantages and disadvantages:

• Various shape of wall can be achieved with this method (for example, provisions for other lines).

• Diaphragm walls are applicable to any ground conditions. The range of construction is wide and the application goes from silt to rock.

• Inner city environment: diaphragm wall and secant piles are commonly used in congested areas. They can be installed in close proximity to existing structures with minimal loss of support to existing foundations. In addition, construction dewatering is not required, so there is no associated subsidence.

• Excellent waterproof: Over cutting joint is possible without changing the underground water table level.

• High cost: Diaphragm wall are more expensive than other method. Nevertheless, the cost is lower when diaphragm walls are used as a permanent structure. It is used for projects under construction in Paris (metro line 13 extension) and Toulouse (metro line 1) and for projects under design (Paris metro line 4 and 12 extensions) as a permanent support. Moreover, it has been used on several metro projects with no internal structure inside (including Cairo, Caracas, Milan and Paris)